The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
The use of fossil fuel, such as petroleum or coal, in a combustion system, in particular the fuel in an engine, entails the production of a non-negligible quantity of pollutants which can be discharged through the exhaust and cause damage to the environment. Among these pollutants, nitrogen oxides (called NOx) pose a particular problem since these gases are suspected of being one of the factors contributing to the formation of acid rain and deforestation. Furthermore, NOx are linked to human health problems and are a key element in the formation of “smog” (pollution clouds) in cities. Legislation is imposing ever more strict levels for their reduction and/or their elimination from fixed or mobile sources.
Among the pollutants that the legislation tends to regulate more strictly are soot or other particulate materials resulting from incomplete combustion of fuel, more particularly when the engine is operated in poor mixture, in other words with excess oxygen (air) relative to the stoichiometry of the combustion reaction. Poor mixtures are used in so-called diesel engines, ignited by compression.
Various different depollution means and strategies are employed to control these two categories of pollutants.
To limit the emission of particles, the technology of particle filters is little by little becoming common practice for all vehicles equipped with a diesel engine. This technology consists mainly in forcing the exhaust gas to pass through the porous channels of a ceramic honeycomb structure. The soot filtered in this way is accumulated and then eliminated in a regeneration operation of the filter during which it is burned. To obtain this regeneration, it is however necessary to increase the temperature of the exhaust gas, which is typically obtained by enriching the exhaust gas with fuel (injected directly in the exhaust line or in the combustion chamber of the engine, during the discharge phase of the combustion cycle) and/or by increasing the charge of the engine. A catalytic agent is used to facilitate the combustion of soot. This agent is either permanently deposited in the filter channels, or introduced as an additive with the fuel. This last technology allows for operating at lower temperatures than those required with catalytic filters.
To limit NOx emissions, the main solution implemented in current vehicles is the reduction of emissions at the source, in other words, operating the engine in such conditions that the rate of NOx produced is less than the limit rate. These conditions are obtained by controlling, in a very precise manner, the different parameters of the engine, starting from the parameters of fuel injection and reinjection at admission of part of the exhaust gas, in order to reduce the oxygen concentration favoring the formation of nitrogen oxides.
However, it is not possible to drastically reduce the emissions at the source without limiting certain engine performances. For this reason, different solutions have been proposed for denitrifying exhaust gas. One solution which has provided proof of its effectiveness, specifically for heavy trucks, is the chemical conversion by reduction of nitrogen oxides by means of a reducing agent injected directly in the exhaust line. A post-treatment solution which has provided proof of its effectiveness is the use of ammonia (NH3), such as aqueous urea. Ammonia reacts with NOx as a catalyst to form inert nitrogen N2 and water H2O. This solution is mainly known under its English acronym SCR or “Selective Catalytic Reduction”.
A commonly used reducer is ammonia, stored in the form of urea, whereby the ammonia is obtained by thermolysis/hydrolysis of urea in the exhaust line according to the following reactions:(NH2)2CO→HNCO+NH3: thermolysis at 120° C.  (1)HNCO+H2O→CO2+NH3: hydrolysis at 180° C.  (2)
The SCR catalyst serves to facilitate the reduction of NOx by NH3 according to the 3 following reactions:4NH3+4NO+O2→4N2+6H2O  (3)2NH3+NO+NO2→2N2+3H2O  (4)8NH3+6NO2→7N2+12H2O  (5)
Since ammonia is considered a toxic gas, it is important that the quantity of injected urea is at all times based on the quantity of nitrogen oxides to be treated.
A simple closed loop control based on the information provided by a NOx sensor installed downstream of the NOx trap is excluded for an engine operating predominantly at transitory speeds, such as the engine of an automotive vehicle.
However, the NOx quantity can be estimated on the basis of mapping nitrogen oxide emissions as a function of the engine operating conditions, in other words, essentially as a function of speed and torque requirements.
In practice however, the precise adjustment of the quantity of urea to be injected poses numerous difficulties. Indeed, the ammonia available for the reaction is the ammonia “stored” at any given time in the catalyst. The higher the temperature of the exhaust gas, the lower the ammonia storing capacity of the catalyst, since a desorption reaction is competing with an adsorption reaction. On the other hand, this temperature increase tends to promote the kinetics of the reaction, and therefore favors reducing reactions. In these conditions, emissions are difficult to control precisely.
It is therefore desirable to detect in real time abnormal operation of the SCR system, without necessarily considering an instantaneous drift of emissions as a sign of such dysfunction.
One of the difficulties in estimating the proper operation of the SCR system is that the exact nature of the gas to be treated is not very well known. Indeed, the NOx consists in part of nitrogen monoxide and in part of nitrogen dioxide, to mention only the two principal substances. The treatment capacity depends on the ratio of these two substances.
This ratio is very difficult to measure even on an engine test bench, since most of the analysis means proceed in the first stage with total oxidation of nitrogen oxides. It is evident then that it is even more difficult to evaluate this ratio in real time in a vehicle, since even the aging, for instance, of certain engine elements can lead to a drift of this ratio.